Traditional metal matrix composite materials are known to be extremely difficult to handle and to process into finished parts. For example, flat sheets employing a thermoplastic binder to hold the metal powders in place during handling have been used to ultimately allow suitable processing into the finished product. Alternative means such as liquid molding into preforms have been used. Also, injection molding techniques may be used for complex-shaped parts in which reinforcements are in the form of chopped whiskers, but are not useful for continuous fibers without suitable preforms. Unfortunately, these known techniques require great effort and added expense, thereby making such metal matrix materials commercially unattractive for widespread adoption in industrial applications. The present invention avoids these difficulties by providing a material which is both easy to handle and easy to form into complex shapes by weaving, knitting or braiding. The instant invention allows the use of continuous fibers, the formation of net-shape parts without the use of a preform and with the metal already distributed throughout the part without having to infiltrate a pre-existing form, and shaping of the part at room temperature with higher temperatures being necessary only at the time of consolidation into the finished part.
Metal matrix composites are typically made from low density metals such as aluminum, titanium, or magnesium which are reinforced with high strength and stiffness fibers such as boron or silicon carbide or with whiskers of materials such as aluminum oxide, boron carbide or silicon carbide. As with polymer matrix composites, physical properties are maximized when continuous fibers, instead of chopped or whisker fibers, are used in the reinforcement. Because of the high temperatures needed in the processing of metals, metal matrix composites are normally not processed in the same manner as polymer matrix composites, except as in the manner described herein in accordance with the instant invention.
A process for making thermoplastic sheathed thermoplastic powder-impregnated fiber roving (i.e., a polymer matrix composite material) is described in U.S. Pat. No. 4,614,678. This patent describes a process whereby fiber rovings or tows are passed through a fluidized bed of thermoplastic powder wherein electrostatic charges cause the thermoplastic powder to adhere to the fibers. Subsequently, the roving is extruded or dip-coated with a solution or dispersion to provide a continuous thermoplastic outer sheath. However, such a thermoplastic powder-impregnated fiber material has relatively low mechanical and thermal properties as compared to a metal matrix composite material in accordance with the present invention. Moreover, the polymer contained in the sheath becomes integrated into the final structure.
Metal matrix composites have several performance advantages over polymer matrix composite materials. Metals generally have higher mechanical and thermal properties than polymers. Further, metals are typically tougher (i.e, a higher fracture toughness) than thermoplastic resins. While a lower density is usually associated with polymer matrix composites, these materials may not be capable of withstanding extreme environmental conditions (high heat, humidity) as can metal matrix composite materials. Metal matrix composites are thus preferred in high temperature applications including, but not limited to, jet engine fan blades, satellite structures, or engine components. Metal matrix composites are also preferred over polymer matrix composites where the material is exposed to organic solvents which would swell or dissolve a polymer matrix material.
Primarily due to differences in thermal properties, it is generally not considered practical to combine polymers along with metals into the same composite form, especially where continuous fibers and/or continuous metal phases are desired. Surprisingly, the instant invention overcomes such limitations on metal/polymer combinations in the manner set forth hereinbelow.
Further, the instant invention allows for protection of the metal impregnated fiber bundle by the jacket surrounding said assembly. In weaving, knitting or braiding operations, considerable potential exists for the fiber bundles to become damaged due to wear and abrasion, particularly where complex shapes are involved. The jacket protects the fiber bundles from such wear and abrasion.